This invention relates to a potential measurement device for measuring potential while irradiating a sample with an electron beam and in particular to a potential measurement device for measuring variations in potential with respect to time (potential waveform) at a fine place (for example, circuit node) in an LSI by using a scanning electron microscope.
It is known that it is possible to measure the potential at a place irradiated with an electron beam by adding a device for analyzing the energy of secondary electrons to a scanning electron microscope (JP-B-47-51024).
FIG. 2A shows this principle. A control grid 5 is disposed between a sample 1 to be tested and a secondary electron detector 4 mounted opposite thereto. This control grid 5 forms a potential barrier for discriminating energies of secondary electrons 3 emitted by irradiation of the sample with an electron beam 2. FIG. 2B is a scheme for explaining the function of this potential barrier. In the case where there were disposed no control grid 5 above the sample, all the secondary electrons would be detected by the secondary electron detector 4. Energies of the secondary electrons emitted by the sample 1 at the earth potential are distributed as indicated by A in FIG. 2B. When the potential of the sample 1 is -5 V, the energy distribution is indicated by B. When the control grid 5 is disposed and a voltage of -5 V is applied thereto, detected secondary electrons are restricted to those having energies higher than 5 eV. Therefore the output of the secondary electron detector varies, depending on the potential of the sample. Since the output depends on the potential of the sample in this way, it is possible to know the potential of the sample 1 by using the output of the secondary electron detector contrariwise.
FIG. 3 illustrates the construction of a prior art voltage measurement device. In this example a hemi-spherical control grid 5 is disposed above an objective lens 6 composed of a magnetic path 12 and a coil 11. The electron beam 2 is focused by the objective lens 6 and the sample 1 is irradiated therewith. Secondary electrons 3 are attracted and accelerated upward by a positive potential supplied to an extraction grid 7 by a positive voltage source 8. Accelerated secondary electrons are subjected to the focusing action of the objective lens 6. They are collected to a focal point and diverged again after having passed therethrough, as indicated in FIG. 3. Then their energies are analyzed by the control grid 5 by a normally negative voltage source 18. The center of the sphere constituting the control grid 5 and the focal point of the secondary electrons are in agreement with each other so that the diverged secondary electrons are projected perpendicularly to the control grid 5. This adjustment is effected by varying the potential supplied to the extraction grid 7 by the positive voltage source 8. The potential measurement device of this structure has a feature that the focal length of the objective lens 6 is short. However, when the trajectory of the secondary electrons was analyzed, it was found that the diameter of the secondary electron beam is only 3-4 mm, when it has reached the control grid 5. In the control grid 5 there is formed an aperture 13, through which the irradiation electron beam passes. The size of the aperture is about 2 mm.phi. and it was found that this is too great with respect to the spread of the secondary electron beam. In the neighborhood of the aperture, since the electric field is disordered, secondary electrons are projected to the control grid 5 not perpendicularly, which gives rise to measurement errors. In addition the detection efficiency is lowered, because the electrons entering the aperture 13 are not detected. However, from the constructional point of view, it is difficult to reduce further the size of the aperture 13. This is because shield grids 9 should be disposed before and behind the control grid 5 and in addition a cylinder 10 should be introduced in the aperture 13, as indicated in FIG. 3. A constant positive potential is given to these shield grids 9 and the cylinder 10 by a voltage source 24 so that it is prevented that the electron beam 2 is deflected by variations in potential at the control grid 5.
Secondary electrons 3, which have overcome the potential barrier formed by the control grid 5, are detected by secondary electron detectors, each of which consists of a scintillator 14, a light guide 15 and a photomultiplier 16. The positive potential by voltage sources 17 plays the role of attracting secondary electrons to the scintillator 14. In FIG. 3 the portion generating the electron beam 2, scanning coils, etc. are omitted.